I. Field of the Invention
This invention relates to underreamers and expansible cutters and, in particular, to an underreamer drill bit with a conical cutter having an improved cutter mounting.
II. Description of the Prior Art
Underreamers, expansible cutters and hole openers have been used for enlarging well bores either connected to a drill string above the drill bit or in the string without any drill bit on the lower end of the string. U.S. Pat. No. 3,386,521 is an example of the combined drill bit and underreamer and U.S. Pat. No. 3,171,503 is an example of an underreamer on the lower end of the drill string. U.S. Pat. No. 4,533,003 illustrates an improved means for securing a cutter to an underreamer arm.
Conical cutters have been used in drill bits and underreamers. Such conical cutters have been installed on a pin which extends downwardly and inwardly from the lower end of the underreamer arms. Roller bearings and balls have been used in mounting the conical cutters on the arms as shown in U.S. Pat. No. 2,641,447. Pins or balls locked in a groove between the arm and the interior of the conical cutter have been used to secure the cutters on their pins as shown in U.S. Pat. Nos. 2,661,447; 3,998,500; and 2,519,716. U.S. Pat. No. 4,161,343 discloses the use of a friction bearing and a thrust button between the pins and the cutters. U.S. Pat. No. 3,998,500 suggests the use of a bolt to secure a bearing sleeve on the pin. U.S. Pat. No. 4,478,299 shows a variety of bearings on the pin including a split bushing arrangement.
The amount of cutting that can be obtained from a conical cutter on an underreamer has been limited by the time in cutting service during which the cutter rotates relatively freely and this is a function of the efficiency of its mounting.